The effect of luminance uniformity on visibility level in road lighting

This study evaluates the effect of low overall and longitudinal uniformities ratios on visual conditions as measured by visibility level at certain points of the road when the average visibility level on the road surface is adequate. A computer program has been developed to determine criteria for calculation of the road lighting based on visibility level to ensure driver comfort and safety. Five different arrangements, left hand single-sided, opposite, staggered, twin-bracket central and combined twin-bracket central and opposite are examined for 30 different road lighting luminaires. To examine the effect of the luminance uniformity on visibility level, average luminances of 1.0; 1.5; 2.0; 2.5 and 3.0 cd/m2 were obtained by changing the mounting height and luminaire spacing. This study explores the effect that luminance uniformity ratios and the luminaire type have on the visibility level. The uniformity ratios provided should not be lower than the values recommended for road lighting calculations based on the luminance concept. To obviate negative visual conditions, a criterion that restricts the distribution of the visibility level values on the road is needed in addition to the average visibility level and both overall and longitudinal luminance uniformity ratios.